Petrology of high-Mg, low-Ti igneous rocks of the Glenelg River Complex (SE Australia) and the nature of their interaction with crustal melts

Although commonly ascribed a mantle derivation, the nature and origin of the parental magmas of the microgranular enclaves encountered in granitic plutons is typically a matter of deduction. This inevitably imparts uncertainty in petrogenetic models for the host silicic magmas, and, in the extreme, can cast doubt over a mingling origin for the enclaves. In contrast, this paper describes a rare example from southeastern Australia where the near-pristine mafic and felsic end-members involved in magma mingling and microgranular enclave formation are identified in outcrop, and their crustal and mantle heritages, respectively, can be demonstrated. The mafic magmas are represented by unusual, Mg-rich, Ti-depleted quartz melagabbros, quartz meladiorites and melatonalites that have affinity with the boninitic lavas erupted above subduction zones. The distinctive chemical signature of these allows the physiochemical processes culminating in microgranular enclave formation to be traced. The participating felsic end-member is muscovite leucogranite, formed by anatexis of metasedimentary rocks near the current exposure level. Interpenetration between these magmas produced mafic cumulates, evolved, variably hybridised derivatives and dispersed microgranular enclave swarms throughout the leucogranite. Localised mixing generated hornblende tonalite, though the widespread occurrence of dykes of this material indicate that mixing was extensive at depth. These relationships suggest that high-Mg magmas may have a key role in the formation of granitic bodies in subduction environments, and in explaining the geochemical variation shown by these plutons.